Eph receptor A4 (EphA4) is a member of the receptor tyrosine kinase family and is a molecule regulating postsynaptic morphogenesis. It is known that knockout of EphA4 or expression of an EphA4 dominant-negative mutant causes a reduction in the number of spines, which are small thorn-like protrusions found on dendrites, and also makes their shape slender (Non-patent Document 1). It is generally proposed that the processes of memory and learning are reflected in the number and/or morphology of spines.
Recent studies have clarified that this EphA4 undergoes γ-secretase-mediated cleavage and the cleaved intracellular fragment activates a small GTP-binding protein, Rac, to thereby promote spine formation (Patent Documents 1 and 2, and Non-patent Document 2). Substrates of γ-secretase are first cleaved in their ectodomains by another protease and then cleaved by γ-secretase. It is known that cleavage of the EphA4 ectodomain is induced in a neuronal activity-dependent manner (Non-patent Document 2). On the other hand, in this series of cleavage processes, the first ectodomain cleavage reaction is known to be a rate-limiting step.
The MMP (matrix metalloproteinase) family, which includes enzymes cleaving off the ectodomain of EphA4, is divided into the classical MMP family and the ADAM family, and there are nearly 50 members in total. Many substrates of γ-secretase, such as Notch and APP, are known to be cleaved by ADAM protease (Non-patent Document 3). Among molecules classified as classical MMPs in the MMP family, MMP-2 and MMP-9 belonging to the gelatinase family are known to be highly expressed in the central nervous system, activated in a neuronal activity-dependent manner, and involved in spine formation (Non-patent Document 4).
However, there is no report at all of the relationship between the gelatinase family and EphA4, and it has never been clarified whether EphA4 is cleaved by gelatinase family molecules.